Super duplex stainless steel having excellent yield strength and impact toughness and menufacturing method therefor

ABSTRACT

Provided is super duplex stainless steel having excellent yield strength and impact toughness, wherein a reduction ratio and a heat treatment temperature are controlled so as to improve mechanical properties. The super duplex stainless steel having excellent yield strength and impact toughness is thick super duplex stainless steel having a thickness of 30 mm or greater, and includes, in weight %, Cr: 24% to 26%, Ni: 6.0% to 8.0%, Mo: 3.5% to 5.0%, N: 0.24% to 0.32%, and the remainder being Fe and inevitable impurities, wherein a microstructure includes a ferrite phase, an austenite phase and a secondary austenite phase, and grain size is 25 μm or less.

TECHNICAL FIELD

The present disclosure relates to super duplex stainless steel and amethod for manufacturing the same, and in particular, to super duplexstainless steel having excellent yield strength and impact toughness,wherein a reduction ratio and a heat treatment temperature arecontrolled so as to improve mechanical properties.

BACKGROUND ART

Generally, super duplex stainless steel (UNS 532750) containing 24% to26% of chromium (Cr), 6.0% to 8.0% of nickel (Ni), 3.0% to 5.0% ofmolybdenum (Mo) and 0.24% to 0.32% of nitrogen (N) is dual-phasestainless steel formed with a dual-phase structure of austenite andferrite, and has been used as materials of desulfurization facilitiesand seawater pipes with very excellent acid resistance and mechanicalproperties.

A matrix structure of such super duplex stainless steel has a structureproperty of a ferrite phase and an austenite phase being formed in anequal ratio. Moreover, super duplex stainless steel has great advantagesof exhibiting higher strength compared to austenitic stainless steel andexhibiting excellent resistance for pitting corrosion for chloride ionsand stress corrosion cracks.

However, super duplex stainless steel contains large quantities ofchromium (Cr) and molybdenum (Mo) for securing acid resistance, andtherefore, when maintained in a 750° C. to 850° C. region, causes aproblem of degrading product qualities such as strengthening brittlenessby readily producing a sigma phase, and significantly reducing acidresistance.

Such a sigma phase is very quickly produced in a specific temperaturerange (750° C. to 850° C.), and therefore, when annealing super duplexstainless steel, being delayed in a specific temperature range thatreadily produces a sigma phase needs to be avoided by controlling atemperature raising rate.

In view of such a problem, “Method for continuous annealing of superduplex stainless steel with excellent impact toughness and coil shape(Korean Patent Application Laid-Open Publication No. 10-2013-0034350)”and the like specifically disclose a method of avoiding a temperaturezone readily producing a sigma phase by raising a temperature from 600°C. to an annealing temperature at a temperature raising rate of 10° C./sor higher, and maintaining the temperature at 1,060° C. to 1,080° C.

The annealing method may be normally used in a hot rolled coil having athickness of 8 mm or less, however, the same heat treatment method mayalso be used in a thick plate having a thickness of 10 mm or greater.However, there is a problem in that a phenomenon not satisfying 0.2%off-set yield strength of 550 MPa or greater over a plate with variousthicknesses from 5 mm to 50 mm frequently occurs.

DISCLOSURE Technical Problem

The present disclosure has been made in view of the above, and isdirected to providing super duplex stainless steel having excellentyield strength and impact toughness, wherein a reduction ratio and anannealing condition are controlled so as to improve mechanicalproperties when manufacturing thick super duplex stainless steel, and amethod for manufacturing the same.

Technical Solution

Super duplex stainless steel having excellent yield strength and impacttoughness according to one embodiment of the present disclosure relatesto thick super duplex stainless steel having a thickness of 30 mm orgreater, and includes, in weight %, Cr: 24% to 26%, Ni: 6.0% to 8.0%,Mo: 3.5% to 5.0%, N: 0.24% to 0.32%, and the remainder being Fe andinevitable impurities, wherein a microstructure includes a ferritephase, an austenite phase and a secondary austenite phase, and a grainsize is 25 μm or less.

The super duplex stainless steel has the yield strength of 550 MPa orgreater.

A sum of the yield strength and the impact toughness of the super duplexstainless steel is 750 or greater.

A method for manufacturing super duplex stainless steel having excellentyield strength and impact toughness according to one embodiment of thepresent disclosure includes casting of preparing a slab including, inweight %, Cr: 24% to 26%, Ni: 6.0% to 8.0%, Mo: 3.5% to 5.0%, N: 0.24%to 0.32%, and the remainder being Fe and inevitable impurities; hotrolling of hot rolling the slab to prepare a thick plate having athickness of 30 mm or greater; temperature raising of raising atemperature of the thick plate to an annealing temperature toprecipitate a CrN phase inside a ferrite phase, and precipitating asigma phase and a secondary austenite phase around the CrN phase; andannealing of keeping the secondary austenite phase inside the ferritephase while solid dissolving the sigma phase and the CrN phase in theferrite phase.

The temperature raising is raising the temperature from 700° C. to theannealing temperature at a rate of at a rate of 0.11° C./s to 0.17°C./s.

The annealing anneals for 20 minutes to 60 minutes at a temperature of1020° C. to 1060° C.

The hot rolling is rolling with a reduction ratio of 80% or greater sothat a grain size of a microstructure becomes 25 μm or less.

Advantageous Effects

According to embodiments of the present disclosure, effects of enhancingmechanical properties such as yield strength and impact toughness ofthick super duplex stainless steel are obtained by inducing CrN phaseprecipitation and facilitating secondary austenite phase formationinside a ferrite phase.

DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing formation behaviors of a sigma phase and a CrNphase depending on a temperature raising rate when annealing.

FIG. 2 shows pictures of a microstructure at temperatures of 800° C.,1000° C. and 1040° C. depending on a temperature raising rate.

FIG. 3 is a diagram showing a behavior of precipitate depending on anannealing temperature and an annealing time, and its microstructure.

FIG. 4 is a graph showing yield strength and impact toughness dependingon an annealing condition.

FIG. 5 is a graph showing a relation between a thick plate thickness(reduction ratio) and a grain size.

FIG. 6 shows pictures comparing microstructures of super duplexstainless steel having excellent yield strength and impact toughnessmanufactured according to one embodiment of the present disclosure and acomparative sheet.

MODE FOR DISCLOSURE

Hereinafter, preferred embodiments of the present disclosure will bedescribed in detail with reference to the accompanying drawings,however, the present disclosure is not restricted or limited by theembodiments. For reference, in describing the present disclosure,specific descriptions on related known technologies may not be includedwhen they may unnecessarily evade the gist of the present disclosure, orcontents considered to be obvious to those skilled in the art may not beincluded.

Super duplex stainless steel having excellent yield strength and impacttoughness according to one embodiment of the present disclosureincludes, in weight %, Cr: 24% to 26%, Ni: 6.0% to 8.0%, Mo: 3.5% to5.0%, N: 0.24% to 0.32%, and the remainder being Fe and inevitableimpurities.

Hereinafter, reasons for numerically limiting the content of thecomponents according to embodiments of the present disclosure will bedescribed.

Cr: 24 wt % to 26 wt %

Chromium (Cr) is a ferrite-stabilizing element, and is an essentialelement for securing acid resistance as well as performing a main rolein securing a ferrite phase. Acid resistance increases when the chromium(Cr) content increase, however, when added in excess of greater than26%, the content of austenite-forming elements such as high-pricednickel (Ni) increases for maintaining a phase fraction, and as a result,manufacturing costs increase.

Accordingly, the chromium (Cr) content is preferably limited to a rangeof 24 wt % to 26 wt %.

Ni: 6.0 wt % to 8.0 wt %

Nickel (Ni) is an austenite-stabilizing element together with manganese(Mn), copper (Cu) and nitrogen (N), and performs a main role inincreasing austenite phase stability. Accordingly, the content islimited to 6.0 wt % to 8.0 wt % for maintaining a phase fraction of theferrite phase and the austenite phase.

Mo: 3.5 wt % to 5.0 wt %

Molybdenum (Mo) is an element very effective in improving acidresistance while stabilizing ferrite together with chromium (Cr), buthas a disadvantage of being very high-priced. Accordingly, themolybdenum (Mo) content is preferably limited to 3.5 wt % to 5.0 wt %.

N: 0.24 wt % to 0.32 wt %

Nitrogen (N) is an element greatly contributing to austenite phasestabilization together with carbon (C) and nickel (Ni), and, as one ofthe elements causing thickening to the austenite phase during annealing,an increase in the acid resistance and high strengthening may beobtained concomitantly when increasing the nitrogen (N) content,however, when the nitrogen (N) content is excessive, surface defectscaused by the generation of nitrogen pores may be induced during castingdue to an excessive nitrogen (N) solid solubility, and therefore, thenitrogen (N) content is preferably limited to 0.24 wt % to 0.32 wt %.

In the super duplex stainless steel having excellent yield strength andimpact toughness according to one embodiment of the present disclosure,a grain size of a microstructure including a ferrite phase, an austenitephase and a secondary austenite phase is preferably formed as 25 μm orless.

In addition, yield strength is 550 MPa or greater, and a sum of yieldstrength and impact toughness is 750 or greater.

Meanwhile, a method for manufacturing super duplex stainless steelhaving excellent yield strength and impact toughness according to oneembodiment of the present disclosure includes a casting step ofpreparing a slab by continuously casting molten steel having theabove-mentioned composition, a rolling step of hot rolling the slab toproduce a thick plate, a temperature raising step of heating the thickplate, and an annealing step.

In the present disclosure, when annealing the super duplex stainlesssteel having both an austenite phase and a ferrite phase, a temperatureraising rate, annealing temperature and time, and a reduction ratio arecontrolled to control a microstructure, and more specifically, bycontrolling a temperature raising rate in the temperature raising step,precipitation of a CrN phase is induced during a temperature rise, andthen precipitation of a sigma phase and a secondary austenite phase isinduced around the CrN phase, and as the sigma phase precipitated in thetemperature raising step is solid dissolved inside the ferrite bycontrolling annealing temperature and time in the annealing step, thesecondary austenite phase remains inside the ferrite phase.

FIG. 1 is a graph showing formation behaviors of the sigma phase and theCrN phase depending on the temperature raising rate when annealing, andFIG. 2 shows pictures of a microstructure at temperatures of 800° C.,1000° C. and 1040° C. depending on the temperature raising rate.

As shown in FIG. 1 and FIG. 2, the temperature raising step according toone embodiment of the present disclosure is preferably raising thetemperature from 700° C. to the annealing temperature having atemperature range of 1030° C. to 1050° C. at a rate of 0.11° C./s to0.17° C./s.

This is due to the fact that a sigma phase is capable of being formedaround the CrN phase while finely precipitating the CrN phase inside theferrite phase.

In other words, when the temperature raising rate is greater than 0.17°C./s, CrN phases are not formed inside the ferrite phase near 800° C.,and even when the temperature is raised to 900° C. to 1000° C., stablesigma phase and secondary austenite phase are formed at an interfacebetween the ferrite phase and the austenite phase, and an effect ofachieving a microstructure may not be obtained.

Meanwhile, when the temperature raising rate is 0.17° C./s or less, CrNphases are finely formed inside the ferrite phase near 800° C., and theCrN phases formed herein act as a nucleation site leading to a formationof sigma phases and secondary austenite phases around the CrN phases aswell as at an interface of the austenite/ferrite phases, and as aresult, a microstructure may be obtained.

FIG. 3 is a diagram showing a behavior of precipitate depending on theannealing temperature and the annealing time, and its microstructure,and FIG. 4 is a graph showing yield strength and impact toughnessdepending on the annealing condition.

As shown in FIG. 3 and FIG. 4, the annealing step according to oneembodiment of the present disclosure is carried out for 20 minutes to 40minutes at a temperature of 1020° C. to 1060° C., and more preferably,the annealing step of the present disclosure varies the annealing timedepending on the annealing temperature.

When the annealing temperature is from 1030° C. to 1050° C., theannealing time is from 20 minutes to 40 minutes, when the annealingtemperature is from 1020° C. to 1030° C., the annealing time is from 40minutes to 60 minutes, and when the annealing temperature is from 1050°C. to 1060° C., the annealing time is from 5 minutes to 20 minutes.

As a result, by increasing the annealing time even when the temperatureis low, a microstructure is obtained by keeping the secondary austenitephase inside the ferrite phase while solid dissolving the sigma phaseinside the ferrite phase, and even with a tendency of the sigma phaseand the secondary austenite phase becoming a solid solution as theannealing temperature increases, an effect of achieving a microstructureis obtained by keeping the secondary austenite phase inside the ferritephase through shortening the annealing time.

FIG. 5 is a graph showing, when producing a thick plate by rolling a 150mm slab, a relation between the thick plate thickness (reduction ratio)and a grain size, and FIG. 6 shows pictures comparing microstructures ofthe super duplex stainless steel having excellent yield strength andimpact toughness manufactured according to one embodiment of the presentdisclosure and a comparative sheet.

In the hot rolling step according to one embodiment of the presentdisclosure, a reduction ratio of the slab is preferably 80% or greater.

As shown in FIG. 5 and FIG. 6, it is seen that, when the slab having athickness of 150 mm is rolled to a thick plate having a thickness of 10mm to 35 mm, a grain size increases as a thickness of the thick plateincreases.

Accordingly, a thick steel plate having a thickness of 30 mm or greaterhas yield strength reduced to 550 MPa, and does not satisfy the ASTMstandards. This may be improved through a method of controlling amicrostructure, however, by using a reduction ratio of 82.5%, yieldstrength may be enhanced while forming a grain size of a microstructureas 25 μm or less.

The super duplex stainless steel having excellent yield strength andimpact toughness according to one embodiment of the present disclosuremay have a thickness of 30 mm or greater. In other words, the presentdisclosure may be useful for a thick steel plate. The upper limit of thethickness is not particularly limited, and for example, may be 100 mm,70 mm or 50 mm.

Hereinafter, a method of controlling a structure of the super duplexsteel having excellent yield strength and impact toughness according toone embodiment of the present disclosure will be described in detailwith reference to examples.

For securing yield strength of 580 MPa or greater and excellent impacttoughness while having overall properties in the super duplex steel, theinventors of the present disclosure formed a CrN phase during heattreatment, and then finely precipitated a sigma phase and a secondaryaustenite phase inside a ferrite phase, by controlling a temperatureraising rate to 0.11° C./s to 0.17° C./s or lower during annealing.

Then, annealing was carried out for 20 minutes to 60 minutes in atemperature range of 1020° C. to 1060° C. to solid dissolving all thesigma phase while keeping the secondary austenite phase inside theferrite phase, and as a result, yield strength and impact properties ofa thick plate having a thickness of 30 mm or greater were both improved.

TABLE 1 Process Variables Slab Thickness Temperature Annealing(Reduction Raising Rate Temperature Annealing Category Ratio) (° C./s)(° C.) Time (min) Note A 77% 1.3 1000 20/40/60 Comparative B 77% 1.31020 20/40/60 Example C 77% 1.3 1040 20/40/60 D 77% 1.3 1060 20/40/60 E77% 1.3 1080 20/40/60 F 77% 0.66 1000 20/40/60 G 77% 0.66 1020 20/40/60H 77% 0.66 1040 20/40/60 I 77% 0.66 1060 20/40/60 J 77% 0.66 108020/40/60 K 77% 0.33 1000 20/40/60 L 77% 0.33 1020 20/40/60 M 77% 0.331040 20/40/60 N 77% 0.33 1060 20/40/60 O 77% 0.33 1080 20/40/60 P 77%0.17 1000 20/40/60 Q 77% 0.17 1020 20/40/60 R 77% 0.17 1040 20/40/60 S77% 0.17 1060 20/40/60 T 77% 0.17 1080 20/40/60 U 82.50%   0.17 100020/40/60 V V1 82.50%   0.17 1020 20 V2 40 V3 60 Example W 82.50%   0.171040 20/40/60 X X1 82.50%   0.17 1060 20 X2 40 Comparative X3 60 ExampleY 82.50%   0.17 1080 20/40/60

Table 1 shows a slab thickness (reduction ratio), a temperature raisingrate, an annealing temperature and an annealing time for variousexamples and comparative examples.

A steel to Y steel that are examples and comparative examples wereheated at a rate of 5° C./s to 700° C., and heated at temperatureraising rates of 1.3° C./s, 0.66° C./s, 0.33° C./s and 0.17° C./s from700° C. to an annealing temperature, and the annealing temperature was1000° C., 1020° C., 1040° C., 1060° C. and 1080° C., and the annealingtime was for 20 minutes, 40 minutes and 60 minutes each, and watercooling was carried out after the heat treatment.

TABLE 2 Temperature Reduction Raising Annealing Annealing SecondaryAverage ratio Rate Temperature Time CrN Austenite Grain Category (%) (°C./s) (° C.) (min) Phase Phase Size Note A to J 77 1.3 to 0.66 1000 to1080 20 to 60 X X 41 Comparative Example K to N 77 0.33 1000 to 1060 20to 60 ◯ ◯ 33 O 77 0.33 1080 20 to 60 ◯ X 38 P to S 77 0.17 1000 to 106020 to 60 ◯ ◯ 30 T to U 77 0.17 1080 20 to 60 ◯ X 36 V to X 82.5 0.171000 to 1060 20 to 60 ◯ ◯ 22 Example Y 82.5 0.17 1080 20 to 60 ◯ X 26Comparative Example

Table 2 shows changes in the microstructure occurring during atemperature raising process when carrying out hot rolling and heattreatment under the conditions described in Table 1.

As shown in Table 2, it was identified that a CrN phase was not formedduring the temperature raising process in A steel to J steel having atemperature raising rate of 0.66° C./s to 1.3° C./s, and a secondaryaustenite phase was not formed inside a ferrite phase as well resultingin the coarsening of the grain size, which is outside the scope of thepresent disclosure.

Meanwhile, in K steel to N steel, a CrN phase was finely formed inside aferrite phase in the temperature range of 700° C. to 800° C. during thetemperature raising process as the temperature raising rate becomes lowof 0.33° C./s, and a secondary austenite phase remained inside theferrite phase in the temperature range of 1020° C. to 1060° C.

Similar to K steel to N steel, a CrN phase was formed in the case of Osteel, however, a secondary austenite phase was solid dissolved and notprecipitated as the annealing temperature exceeded 1080° C.

P steel to U steel had a temperature raising rate of 0.17° C./s, whichtends to be similar to K steel to O steel, however, as the amount of CrNphase precipitation increased, the remaining secondary austenite phaseincreased as well.

In addition, A steel to U steel had a reduction ratio of 77% resultingin the coarsening of the final microstructure grain, and the size becamegreater than 25 μm, which is outside the scope of the presentdisclosure.

Meanwhile, in V steel to X steel satisfying the embodiments of thepresent disclosure with a reduction ratio of 82.5%, a temperatureraising rate of 0.17° C./s, and an annealing temperature of 1020° C. to1060° C., a secondary austenite phase remained inside a ferrite phase inthe temperature region of 1020° C. to 1060° C. while properlyprecipitating a CrN phase in the temperature raising process in some ofV steel and X steel and all of W steel depending on the annealing time,and most fine structures were secured.

Meanwhile, it was seen that, like T steel, Y steel was outside the scopeof the present disclosure with a secondary austenite phase being soliddissolved with an annealing temperature of 1080° C.

TABLE 3 Reduction Temperature Annealing Annealing Grain Yield ImpactRatio Raising Temperature Time Size Strength Toughness (A+ Category (%)Rate (° C./s) (° C.) (min) (μm) (A) (B) B) Note T 77 0.38 to 0.17 108040 36 to 43 536 172 708 Comparative Example R 77 0.33 to 0.17 1040 40 28to 33 569 187 756 W 82.5 0.33 to 0.17 1040 40 21 to 24 585 193 778Example

Table 3 shows properties for representative steel types (T, R, W) ofTable 2.

Herein, as for the yield strength, a JIS 5 tensile specimen wascollected in a 90° direction of the rolling direction and a tensile testwas carried out at a crosshead speed of 20 mm/min at room temperature.

In R steel, the grain became coarse with a reduction ratio of 77%, andthe size was greater than 25 μm, a standard value, and particularly in Rsteel, the yield strength was 536 MPa, which was less than 550 MPa, astandard value, and a sum of the yield strength and the impact toughnesswas 708 MPa, which was also less than 750 MPa, a standard value, and itwas seen that yield strength and impact toughness properties were notenhanced.

In addition, in T steel, the yield strength and a sum of the yieldstrength and the impact toughness satisfied the standard values,however, the grain size was greater than 25 μm, a standard value, with areduction ratio of 77%.

Meanwhile, in W steel, the reduction ratio was 82.5%, and the annealingtemperature, the annealing time and the temperature raising ratesatisfied the scope of the present disclosure, and as a result, thegrain size was fine with 25 μm or less, and the yield strength and theimpact toughness were enhanced with the yield strength being 585 MPa anda sum of the yield strength and the impact toughness being 778 MPa, andit was identified that mechanical properties were enhanced compared tothe comparative sheets.

As described above, the present disclosure has been described withreference to preferred embodiments, however, it is to be understood thatthose skilled in the art may diversely modify and change the presentdisclosure within the scope that does not depart from ideas andterritories of the present disclosure described in the attached claims.

1. Super duplex stainless steel having excellent yield strength andimpact toughness comprising, as thick super duplex stainless steelhaving a thickness of 30 mm or greater, in weight %, Cr: 24% to 26%, Ni:6.0% to 8.0%, Mo: 3.5% to 5.0%, N: 0.24% to 0.32%, and the remainderbeing Fe and inevitable impurities, wherein a microstructure includes aferrite phase, an austenite phase and a secondary austenite phase, and agrain size is 25 μm or less.
 2. The super duplex stainless steel havingexcellent yield strength and impact toughness of claim 1, wherein theyield strength of the super duplex stainless steel is 550 MPa orgreater.
 3. The super duplex stainless steel having excellent yieldstrength and impact toughness of claim 2, wherein a sum of the yieldstrength and the impact toughness of the super duplex stainless steel is750 or greater.
 4. A method for manufacturing super duplex stainlesssteel having excellent yield strength and impact toughness comprising:casting of preparing a slab including, in weight %, Cr: 24% to 26%, Ni:6.0% to 8.0%, Mo: 3.5% to 5.0%, N: 0.24% to 0.32% and the remainderbeing Fe and inevitable impurities; hot rolling of hot rolling the slabto produce a thick plate having a thickness of 30 mm or greater;temperature raising of raising a temperature of the thick plate to anannealing temperature to precipitate a CrN phase inside a ferrite phase,and precipitating a sigma phase and a secondary austenite phase aroundthe CrN phase; and annealing of keeping the secondary austenite phaseinside the ferrite phase while solid dissolving the sigma phase and theCrN phase in the ferrite phase.
 5. The method for manufacturing superduplex stainless steel having excellent yield strength and impacttoughness of claim 4, wherein the temperature raising is raising atemperature from 700° C. to the annealing temperature at a rate of 0.11°C./s to 0.17° C./s.
 6. The method for manufacturing super duplexstainless steel having excellent yield strength and impact toughness ofclaim 5, wherein the annealing is annealing for 20 minutes to 60 minutesat a temperature of 1020° C. to 1060° C.
 7. The method for manufacturingsuper duplex stainless steel having excellent yield strength and impacttoughness of claim 4, wherein the hot rolling is rolling with areduction ratio of 80% or greater so that a grain size of amicrostructure becomes 25 μm or less.